Current wireless communication systems are based on two popular standards: a wide area network (WAN) standard referred to as the Fourth Generation Long Terra Evolution (4G LTE); and a local area network (LAN) standard called Wi-Fi. Wi-Fi operates on shared or unlicensed spectrum as short-range wireless extension of wired broadband systems. The 4G LTE on the other hand provides wide area long-range connectivity using licensed spectrum and dedicated infrastructure such as cell towers and backhauls to connect to the Internet.
As more people connect to the Internet, increasingly chat to friends and family, watch videos, listen to streamed music, and indulge in virtual or augmented reality experience, data traffic continues to grow at unprecedented rates. In order to address the continuously growing wireless capacity need, the next generation LAN and WAN systems are expected to use higher frequencies referred to as millimeter wave spectrum shown in Table 1.
TABLE 1Examples of millimeter wave bandsBands [GHz]Frequency [GHz]Bandwidth [GHz]24GHz24.25-24.450.20024.75-25.250.50028 GHz/LMDS 27.5-28.350.850 29.1-29.250.150 31-31.30.30032GHz31.8-33 1.20037GHz37.0-38.61.60039GHz38.6-40 1.40042GHz42.0-42.50.50047GHz47.2-50.23.00050GHz50.4-52.61.20060GHz57-647.00064-717.00070/80GHz71-765.00081-865.00090GHz92-942.90094.1-95.095GHz 95-1005.000105GHz102-1057.500 105-109.5112GHz 111.8-114.252.450122GHz122.25-123  0.750130GHz130-1344.000140GHz 141-148.57.500150/160GHz151.5-155.512.50155.5-158.5158.5-164 
The two most commonly used unlicensed frequency bands below 6 GHz are the 2.4 GHz band and the 5 GHz band. The 5 GHz unlicensed band offers many hundreds of megahertz (MHz) spectrum as illustrated in FIG. 1. The 2.4 GHz and 5 GHz unlicensed bands are generally used by the local area network (LAN) standard called Wi-Fi. The 4G LTE cellular systems generally use licensed bands below 6 GHz.
Current 4G LTE and LAN networks that utilize time division duplex (TDD) suffer from numerous drawbacks. Since downlink (DL) and uplink (UL) use the same frequency band in TDD-based 4G LTE and LAN, DL signals transmitted by a base station or an access point at high power interfere with UL signals transmitted by client devices at low power to another base station. Similarly, the signals transmitted from one client device connected to a base station will interfere with the signals received by another client device connected to another base station. Consequently, transmitters and receivers must be synchronized so that UL and DL data packets are transmitted in respective time slots to avoid interference. Thus, a client device (e.g., mobile phone) must wait for a UL time slot before commencing UL transmission. Similarly, a base station or access point must wait for a DL time slot before the base station or access point may commence DL transmission. As a result, delay is caused in UL and DL transmissions. Also, hardware including synchronization circuits are necessary to synchronize the client device with the base station or access point, which increases overcall cost of the systems. Furthermore, if the data packets transmitted by a client device or a base station only partially fills the time slots due to the size of the data packets, transmission throughput is reduced because of unfilled or wasted time slots.
In 4G LTE and LAN networks that utilize frequency division duplex (FDD), transmitters and receivers can operate asynchronously because UL and DL frequencies are different. However, because UL and DL frequencies are not widely separated, the transmitter's output may overload the receiver's input. Consequently, transmitters and receivers require high-complexity multi-pole filters to provide significant attenuation to prevent the transmitter's output from overloading the receiver's input.